Continuous production of heattreatable ferrous sections



May 28, 1946.

P. H. HUME E'rAl.A 2,400,932 CONTINUOUS rnonucnou oF HEAT-TREATABLE Fnnnous sEcTIons FIB- 1- 3 Sheets-Sheet l #05m/w TE.

3 Sheets-Sheet 2 P. H. HUME ETAL Original Filed April l2, 1943 CONTINUOUS PRODUCTION OF HEAT-TREATABLE FERROUS SECTIONS May 28, 1946.

y /m H s m F A 1. e Y S n o nllvlo .Q

z m m@ A h mw -E Ef my .:VU MW 45 E S m ix N\ Q Q N\ \N\/.M\ M l WI. N Q W .a on. I l( n m Q P LCIHIIHUI Mlly 28, 1946. P. H. HUME Erm. 2,400,932

"K CONTINUOUS PRODUCTION OF HEAT-TREATABLE FERROUS SECTIONS Original Filed April l2, 1945 3 Sheets-Sheet 3 [5 (fw: PIE- 4- d /4 i L f/ 8\ INVENmRS: Pam/aos( Aff/M5 lfm/#M /FM/W md {D14/#e0 l. P05/N50,

Eff/f? mle/VIK Patented May 28, 1946 CONTINUOUS PRODUCTION OF HEAT- TREATABLE FERROUS SECTIONS Patrick H. Hume, Lakewood, Ohio, William F. McGarrity, Hollidays Cove, W. Va., and Edward L. Robinson,

Pittsburgh, Pa.,

assignors to Carnegie-Illinois Steel Corporation, a corporation of New Jersey Original application April 12, 1943, Serial No.

482,774. Divided and this application December 12, 1945, Serial No. 634,577

1 Claim.

This invention relates to the adjustment of the physical properties of ferrous bodies, especially steel plates, sheets, and strip, in a greatly simplified and economic manner.

This application is a division of application Serial No. 482,774, filed April 12, 1943.

The physical properties of ferrous metals are largely determined, as a general matter, by three fundamentals:

l. Composition 2. Structural constituency 3. Condition of the constituent structure Considered in their order, composition is of the first importance, since it xes the species of steel among the genera of ferrous metals, by predetermining the allotropy and phase relationships characteristic thereof, and by modifying the ferrous constituent directly in consequence of alloying ad tions.

After composition is established, the next in importance is the structural constituency of the specific metal whereby its allotropic and ancillary phases may be adjusted to afford optimum .properties.

The least important (except in non-heattreatable steels), from the standpoint of overall eil'ect on the nal properties, is condition of the structure, dealing with purely physical considerations, such as the size and shape of grains and segregates, and their relative placement, by the adjustment of which relatively slight modifications of the physical characteristics of a steel may be made.

'I'he present invention does not deal with the first fundamental (composition), being, as it is, applicable to ferrous metals of any analysis that is responsive to heat-treatment; nor to the third fundamental, other than indirectly, since considerations of modifying the metallographic condition of the structural constituent in a manner corresponding to the ,present invention have already been dealt with elsewhere, as will become apparent hereinafter. Rather, the present invention treats largely with the second fundamental, and is directed toward the adjustment of the structural constituency of a steel body in such a manner as to preclude the necessity for elaborate heat-treating equipment and processing cycles characteristic of prior art technique.

It is, accordingly, the principal object of the present invention to impart the desired final properties to ferrous bodies, through adjustment of their structural constituency, in a simple, direct and economic manner.

It is a related object to process heat-treatable steels in a continuous manner with reference to some essential prior processing step involving heat, such as hot-reduction of the gauge, whereby effective use of residual heat may be had to develop properties usually associated with separate subsequent heat-treatment in furnaces, or the like.

It is a further object to process steels in accordance with a new technique herein called differential reflex tempering."

Other objects and advantages, including increased production and lowered costs, will become apparent as the description progresses.

Steel sections, worked to gauge by hot or coldreduction, have, in the majority of cases, been subjected to various annealing or normalizing operations, i. e., heat-treatment, to develop the required final properties of the metal, in accordance with conventional practices. This has involved great expenditure in time, labor and equipment which has added materially to the cost per ton of steel so produced, while, at the same time, has curtailed the production thereof. Hot-reduced material, after reduction to gauge, must be cooled, charged into furnaces, reheated and preferentially cooled (quenched), flattened, and, in some cases, heated and attened again before the final properties are imparted. Such practice, especially in plate gauge material involving heavy masses, entails a slow cycle for heating, warpage upon quenching, consequent flattening operations, and many steps which prolong a production cycle into days.

The present invention eliminates the need for heat-treating and processing cycles by utilizing heat residual in the metal from previous processing, e. g., hot-rolling, to obtain the same results continuously, automatically, and substantially in one operation. As a brief outline of a preferred embodiment of the invention, plates, sheets or strips of steel of heat-treatable composition are rolled to gauge on a hot-mill so as to iinish at austenitizing temperature. Such stock is then delivered to a series of specially designed sprays, preferably associated with the run-out table of a continuous hot-mill, where it is cooled in a preferential manner without warpage; all of the stock being taken through the transformation point at approximately equilibrium temperature, or at a. depressed temperature, depending upon the structure sought, or part of the stock being cooled and part being left relatively hot for mutual structural adjistment by residual heat transference when stacked or coiled together (dinerential reflexive tempering).

The invention will be better understood when the following description is considered in conjunction with the accompanying drawings in which:

Figure 1 is a graph showing the effect of different rates of cooling on the transformation temperature of steel as represented by a typical S-curve. Here the temperature of transformation (ordinates) is plotted against the transformation time on a logarithmic-scale (abscissae) to reveal the allotropic structures that may be obtainecqlgion transformation at different levels;

Figure 2 is a diagrammatic plan view of a runout table of a continuous hot-mill indicating the placement of sprays in accordance with one embodiment of the present invention;

Figure 3 is a side elevational view drawn to an enlarged scale of a typical spray unit for application as in Figure 2;

Figure 4 is an end view of the apparatus illustrated in Figure 3 as viewed from the left;

Figure 5 is a schematic representation of the end of a run-out table and a piler illustrative of one method for selectively cooling plates and sheets, according to the present invention; and

Figures 6 and 7 illustrate the principle, described in connection with Figure 5, as applied to strip.

Heretofore, the continuous production of plates, sheets and strip on a tandem hot-mill has been accomplished by heating slabs in furnaces and then advancing them through a series of roll stands (some of which may be reversing) called the roughing train, and, thence, on through another series of stands continuously, which reduce the material to gauge. This latter is referred to as the finishing train, and usually has associated with it a long run-out table at the exit end of the last stand by which the stock is advanced to the coiling or piling mechanisms. Such tables have been constructed of sufficient length to provide for the adequate cooling of the stock to a temperature below that at which welding between contiguous portions of a stack or coil would occur. In some instances, water sprays have been provided to supplement the aircooling for the purpose of assuring that the temperature of the stock is reduced a suflicient amount.

The sprays associated with conventional arrangements of run-out tables are of small capacity, since, for the most part, they are designed to act upon relatively thin-gauge stock, and then only in an auxiliary capacity to assist in the cooling thereof. Heavier gauge material, such as plates, could not be eectively treated with the conventional sprays, and even the thinner gauges have not been cooled with sufficient rapidity to alter the transformation temperature of the steel to any 'material degree. Taking into consideration that temperatures of recrystallization extend to below the transformation range, and, in point of time, are maintained longer, more recent developments have produced a spray of higher efficiency, whereby an adjustment of the condition of the structural constituent of the metal (grain size, dispersion of segregates, etc.) has been possible, and to this extent the physical characteristics of the material have been adjusted and controlled in a, manner usually associated with such condition of the metal. This deals with the third fundamental discussed above, and is the subject of a patent to Herman et al., No. 2,271,372.

In accordance with the present invention, it has been discovered that specially designed sprays, capable of delivering more water at higher velocity, and capable of being controlled to a critical degree far beyond anything contemplated heretofore, can be applied in the production of hot-reduced plates, sheets and strip so as to control the transformation temperature upon cooling from within the austenitic range to impart the desired structural constituency to the metal.

In the accompanying drawings, Figure 1 sets forth a typical S-curve by which it is intended to illustrate the aims of the present invention. It will be understood that the S-curves of steels, being indicative of the time required to effect transformation at different levels of temperature, will vary as the composition varies, thus rendering any graphic representation, in terms of specific temperatures and times applicable to all steels, an impossibility. For this reason no specific temperatures or time intervals have been included in Figure l, since the considerations here are applicable to steels of any heat-treatable composition. However, with the principle understood in general, specific values may be supplied to Figure 1 to satisfy the conditions of a given composition.

By reference to Figure l, several pertinent observations may be made that will be applicable to any steel under treatment. Let it be assumed that a steel body is at a temperature within the austenitic range. Under conditions (slow-cool) tending toward theoretical equilibrium between the external energy of heat and the internal energy of the metal associated with phase retention or transformation, the austenite-to-ferrite transformation would occur somewhere along a substantially straight line marked Aa The more rapidly the cooling is conducted, (i. e., the closer the cooling-rate curve falls to the lefthand side of the graph) the lower the transformation temperature is depressed, as is evidenced by the sloping line I, indicative of the commencement of such phase-change. If the cooling is accomplished with sufiicient rapidity to cool the metal from a temperature above the Ae temperature down to a temperature represented by abscissa Tri in t1 seconds, the lowest transformation point in the upper temperature levelsjfrom the standpoint of the least time and temperature, will have been met. This point (where Tri and t1 intersect in Figure 1), commonly referred to as the nose or elbow of the S-curve, has been designated by the letter N in this figure, and in non-alloy or low-alloy steels is usually attained in a second or less. The cooling rate which, when plotted, defines a curve or line passing tangentially to the nose N of the S-curve is termed the critical cooling rate, since transformation above or below this point occurs in a greater order of time. Therefore, in order to make available those structures resulting from transformation at lower temperatures, the critical cooling rate must be invoked as the maximum rate in point of time admissible to effect this end, lest a. slower rate initiate transformation at a higher temperature.

'I'he second curve 2 of Figure l is representative of the time-temperature variant, at which the transformation, initiated upon intersection of curve I, is completed, after the attainment of which no further adjustment of the component structure of the metal (barring conditioning under the third fundamental) is possible by cooling alone, without reheating. In accordance with this, the curve of Figure 1 has been divided into zones demarking the different structures derived from effecting transformation at different time-temperature relationships. transformed within zone A, and the transformation is completed at such temperature, a substantially uniform structure of coarse pearlite in If austenite is '-nuallwwcr varying amounts, depending upon the carbon content, will be obtained. Transformation begun and completed in zone B will afford a medium arlite structure, while that occurring in Zo will be the smallest, most highly refined pearlitic structure. Below this zone, pearlitic characteristics give way to acicular structures ranging from the softer bainite in zone D down to the hardest martensite in zone E.

For uniformity, transformation should be completed at approximately the temperature level at which it is begun. Where transformation is initiated in one zone, or at a temperature provident of a higher-temperature-phase structure, and the cooling rate is such as to complete the transformation in another zone, or at a temperature at which lower-phase structures will have formed, a mixed or heterogeneous structure will result predominated by that structure derived from a temperature level at which the metal will have stood the longest, short of completing the transformation. Borderline or mixed structures are obtained from interzonal transformation.

Because of the drastic rate of cooling entailed in constituent structure adjustment, inadequate facilities, and immurement by heat-treating habits of long standing, hot rolled material, as produced in accordance with conventional practices, whether subsequently cold reduced or not, has, in every case, had the residual heat of rolling dissipated in such a manner as to require any preferred structural constituency of the metal, other than that occurring between the A. and the Tri temperatures (Figure 1) in accordance with the above, to be acquired in separate heat-treating operations. The present invention, by utilizing the residual heat of rolling in conjunction with special apparatus for the impartation of preferential cooling, has achieved the adjustment of the structural constituency without the need of further heat treatment. The manner by which this is accomplished, in a preferred form of the invention, will be understood by reference to Figures 2 to 7, inclusive, of the drawings.

Figure 2 is a schematic plan view of a hot-mill run-out table to which spray apparatus, in accordance with the present invention, is applied. 'I'he last stand 5 of the continuous hot-mill is shown at the left-hand end of this figure, relative to which the stock' moves into the direction of the arrows. After emerging from the roll stand 5, the stock is passed through a flying shear 6, a descaler 1, and one or more spray units 8, which are designed to give effect to the present invention, and which will be described in greater detail hereinbelow. If strip is being produced, the stock is conducted by a continuation of the roller table 9 to coilers I0, or if plates or sheets are being produced, the stock is conducted beyond the coilers I by a continuation of the roller table 9, through another series of sprays 8, to a stacker or piler unit II, from which they are removed for storage or further processing.

Each of the spray units 8 may comprise an independently controllable series of sprays arranged above and below the roller table in a manner similar to that shown in Figures 3 and 4. Any suitable structure to this end is intended, although the one illustrated has proved to be highly eilicient under conditions of actual operation. In these drawings, the conveyer-table rollers are indicated at 9', relative to which the path line of stock-travel falls tangent to their upper surfaces. A plurality of transversely-disposed spraypipes are horizontally arranged across the rollertable above the pass-line of the work. Each comprises a pipe I2 having a plurality of nozzles I3 positioned approximately ten inches from the upper surface of the work-piece. As illustrated. there are ten (10) cross-pieces I2 carrying seventeen (17) nozzles each. 'I'he cross-pipes I2 are connected by risers I4 to a manifold I5, which, in turn, is connected to the water supply I1 by pipe I8.

correspondingly arranged below the pass-line of the work, between the rollers of the table, is a lower series of spray-devices comprised of crosspipes I2' carrying nozzles I3 disposed in opposition to those of the upper bank. In a similar manner to that already described, the cross-pipes I2 are connected to a manifold I5' by a riser I4' and, through a pipe I6', to the main conduit of supply I1.

These spray-devices are such as to deliver large quantities of water, the temperature of which is adjusted to a uniform temperature to safeguard against seasonal differences, at high velocity to insure that, even from the heaviest plate section, rapid and uniform abstraction of heat will be effected without warping material. To this end, water, under 1000 to 1200 lbs. per square inch pressure, is handled by each unit at the rate of 2800 gallons per minute, and the units are so arranged in connection with the main water supply as to be turned on and of! independently of each other, as by valves I8 (shown in Figure 2), to admit of cooling in any pre-selected manner.

Except on such installations where coiling and n piling can be accomplished at substantially the same location in reference to a run-out table, it is usual to provide separate groups of sprays for material produced in continuous lengths, which will be coiled as at I0 in Figure 2, and for that which is cut into sheets or plates, which will be piled as at II in this same gure. Thus, in Figure 2, one or more of the spray units to the left of coilers I0 may be used in the production of strip, while in the case of sheets or plates, a lesser number or none of these first units will be used to conserve the heat of the material for its longer travel down the table, relying upon the spray units to the right of the coilers I0 to make the final temperature adjustment fairly close to the pilers II.

As a typical example of this invention, armor plate manufactured from steels of S. A. E. specications ranging from 3120 XX to 3325 n, which include steels containing carbon within the range of 0.20 to 0.30%, nickel from 1.25 to 3.50%, and chromium from 1.00 to 1.60%, has been produced in final gauges ranging from to 1" thick. Slabs of such material were preheated so as to leave the last pass of the finishing mill (5 in Figure 2) at temperatures ranging from 1650 to 1750", or, in any case, within the austenitic range. As the 11st plates advanced down the roller table 9 they were sprayed by three of the units 8 to the left cl the coiler I0, and again by the units 8 to the right of the coller I0 adjacent to the piler, at which point they were at a temperature less than the boiling point of water. The structure derived was hard martensite, evidencing transformation in the middle to lower region of zone E, Figure 1, and the plates passed ballistic tests in this condition.

To facilitate subsequent fabrication, it is preferable to reheat the plates to around 600 F. to temper the martensitic structure. In some instances, this has been accomplished by controlling the sprays to reduce the surface temperature of the plates to a temperature in the order of 400, allowing the residual heat in the core of the stock to draw the surface metal to the required temper. The temperature gradient in the heavier sections is one that steadily declines in the central portion of the cross sections to a down-andup (averaging down) gradient at the surface. A sharp decline of temperature at the surface, as a result of direct contact with water, is followed by an increase in temperature as the work-piece moves beyond the spray and thermal equilibrium is restored throughout the mass by redistribution of heat from the hotter interior to the cooler exterior. Instead of the usual homogeneous sorbitic structure normally associated with quenched and drawn armor plate, a composite structure of upper bainite and martensite has been observed, which is attributable to the interrupted quenchand-draw as here applied. I'he steel thus produced, in addition to a high ballistic rating, is possessed of deep hardenability, good weldability, and, under heavy shock test, does not spall, which, particularly in the thinner sections (less than 3,/4"), is a characteristic very hard to obtain. Where softer structures are desired, the cooling may be retarded by controlling the sprays 8, as well as the speed of travel of the plate, to gi-.ve progressively softer structures ranging from martensite up to pearlite, and, in the case of carbon steel, the temperature differential from the surface to the core of the section has been such as to produce a ne pearlitic center and a spheroidized surface on plates subjected to this treatment. 'I'hese are examples of differential reflex tempering.

The final structural constituency is determined by the rate of cooling and its effect upon the austenite-to-ferrite transformation point. If the latteris depressed below the nose N of the S- curve, as shown in Figure 1, ferrite and carbides,

f bainite, martensite, or borderline structures, may

be developed at progressively lower temperatures, depending upon where the transformation curve is intersected. The resulting structure may be retained as is, or may be modified by regular tempering treatment or differential reflex tempering,

The sprays are designed to afford uniform cooling to prevent warpage. They have sufcient force to wipe away steam envelopes which tend to form insulatory sheaths about the plates and cause non-uniform cooling conditions, while the volume of water delivered is such as to abstract the heat required. Any suitable equipment and fluid medium is intended for this purpose. This practice, as distinguished from the older practices in which the steel had to be subjected to several reheatings, including a quench in a. bath, is greatly simplified andexpedited. As has been said, the effect of the bath-quench in the prior practices was to induce severe warpage, which, in turn, required several flattening operations in heavy presses, in the case of plate gauge stock, and roller leveling or cold-passing in the case of strip and sheets. All of this has been eliminated in the present invention, which is outstanding because production schedules for armor plate, previously measured in days, are now measurable in minutes by application of the teachings hereof.

In strips and sheets, too, the vigorous sprayquenching, described above in connection with plate-gauge material, can be applied, though less drastically to conform to the requirements of the lighter sections. Since the masses involved in sheet and strip gauges render controlled abstraction of heat more difficult, there has been developed a differential reflex tempering method, in accordance with the present invention, which, while equally applicable to plate sections and heavier gauges, is ideally adapted to heat-treating strips and sheets in a controlled manner selectively to impart the desired metallographic structure to the metal. This method is illustrated in Figure 5 of the drawings. Successive sheets or plates are alternately quenched (20C) by the spray, to any required lower temperature down to room temperature, while the remainder (20H) is allowed to pass down the table without quenching (or with less quenching) so as to reach the piler l I at a substantially higher temperature than the quenched sections (20C) preferably near or above the austenitic temperature (around 1400o or above). 'Ihe relatively cold and hot sheets or plates, thus derived, are stacked in alternation, hot-cold-hot-cold-hot, etc., to provide a pile 2l of mutually sandwiched hot and cold sheets. It is obvious that the heat in the hot sheets or plates will be transmitted to their colder neighbors to effect a tempering of the quenched structure existing in the latter, while the rate of abstraction of heat from the sheets or plates by the relative cold ones will give an accelerated cooling through the critical temperature range selectively to aord the desired structure depending upon the initial temperature differential between the two sets of sheets. As thermal equilibrium is approached, a soaking temperature uniformly distributed will prevail, allowing the pile slowly to cool to room temperature. Covers, or a. holding furnace, may be utilized to cool the pile more slowly and uniformly.

Different structures are obtainable through different combinations of mass-temperature relationships, e. g., two or more hot sheets may be included for every cold one, or vice versa, thus providing slower or faster cooling for the hot sheets, and higher or lower drawing temperatures for the cold ones, which, in accordance with the transformation as determined by the S-curve of the steel of the composition in question (of which Figure 1 is a typical example), will afford the desired metallographic structure in a predictable and controllable manner. It is obvious that sheets or plates of intermediate temperatures may be interspersed With the extremely hot and cold ones to vary the conditions of transformation and tempering as desired. The order of piling should be noted in order to permit the repiling of sheets and plates into groups of metallographically similar characteristics.

'I'he same idea is adaptable to strips (Figure 6) by quenching one or more continuous lengths (22C) while leaving others hot (22H), superimposing one upon another in any desired relationship, and then coiling together (23) for the attainment of thermal equilibrium. More conveniently, contiguous portions of a single strip 22 may be alternately cooled (as by spraying) and left hot, respectively, so as to provide a strip having hot and cold areas (H and C, Figures 7) in succession. Upon coiling a strip prepared in this manner (23), isothermal conditions are approached, thus adjusting the allotropic condition of the metallic structure of the entire strip in one operation. As an assumed example, the cold sections C of a strip treated in this manner may have been quenched to insure transformation into martensite; i. e., cooled at a rate so as to attain a temperature below Tri in t1 seconds or less for transinnation in zone E, Figure 1. The temperature of the hot sections H upon coiling will be such (e. g., 1400 F.) as to draw the temper of the colder martensitic portions into a structure of, say, sorbite, while the heat exchange attendant upon this tempering lwill effectively quench the hot sections of the strip down to a temperature at which a nodular structure corresponding to sorbite is obtained. The residual heat, after conditions of substantial thermal equilibrium have been attained (in the assumed example, this should fall between 500 to 700 F.), will afford a slow cooling to room temperature from which a homogeneous structure throughout the entire strip is assured.

The pre-established temperature differential, and the ratio of the masses of the hot and cold portions provide adjustable variables for the attainment of any desired nal structures in this coilor stack-tempering arrangement. Stated another way, the mass-temperature differential prevailing between the hotter` and colder portions of a stack, pile, or coil, is such as to effect self- (reflexive) annealing and tempering.

The relationship between mass-temperature and final equilibrium conditions in pack or coil may be expressed by the following equation:

in which E represents the highest theoretical equilibrium temperature; T, the total heat (in calories); M, total mass under treatment, and K, a constant to correct for heat-losses to the air, scale, etc. To simplify the following illustrations, heat has been reduced to its function, temperature, expressed in degrees Fahrenheit, since the individua1 masses involved have been regarded as unity.

Example I To illustrate this, let it be assumed that successive sheets of eutectoid carbon steel, finished to the same gauge and sheared to the same size, are advanced from the last stand (5, Figure 2) of a. continuous hot mill at a temperature of 1678 F. Alternate sheets are quenched to 78 F. (room temperature) while the remainder is allowed to proceed to the piler unquenched. Let it be assumed further that the temperature loss of the hot sheets in traversing the table is about 100 F. Then, with mass as unity, and disregarding the losses which are constant:

The hot sheets will have been reduced from above the transformation (Ae-about 1340 F.) to 828 F. in from one to two minutes, imparting a from coarse-to-ne pearlitic structure, while the cold sheets will have been quenched to the martensitic range, and drawn back to 828 F. to form a sorbitic structure evidencing incipient spheroidization of the cementite.

Emample II The effect of mass may be illustrated by the following in furtherance of Example I:

(A) For each sheet piled at 1578 F., let two sheets be piled at quenched temperature (78). 'I'hen:

The hot sheets will have a ne pearlitic structure, while the quenched sheets will possess a full stressrelieved troostite structure.

(B) As the reciprocal of II(A), let one sheet be quenched, and two sheets unquenched. Then:

In this case, transformation of the hot sheets will be performed close to the Ae temperature to aiorcl coarse pearlite with agglomerated carbides due to the arrested cooling at such a high temperature. The quenched sections will have a strong, ductile, nodular structure of sorbite and spheroidized cementite with exceedingly fine grains.

Examples illustrative of the effects derived from varying the mass-temperature relationships could be multiplied indefinitely. The foregoing, however, are sufficient to illustrate the principle by which any condition of quench, interrupted quench, quench and draw, Austempering (Patent N o. 1,924,099) Purnellizing (application led July 18, 1940, Serial No. 346,224), and related heattreatments, may be accomplished. In the foregoing examples, sheets are intended to apply to any out lengths irrespective of gauge, or contiguous sections of strip, or other long lengths, whether stacked or coiled.

Apart from the obvious advantages of simplicity, economy and expediency, other benets are derived from the automatic aspects of the invention whereby, once the mass-temperature relationships are established empirically for a steel of given composition, a given set of physical properties, in terms of structural consistency, is duplicatable indefinitely. The finishing temperature of the stock, the quantity and temperature of the quenching medium, the masses involved at an appointed temperature, and the speed of stock-travel, all may be selectively varied or xed at will to produce the desired results. The promptness with which the quenching is followed by tempering reduces the losses occasioned by delays sometimes encountered in this interval, thus preventing strain cracks and failures, and destruction of the properties of the metal.

It will be appreciated, as set forth in the following claim, that it is the simplied processing to which the present invention is directed. by whatever means attained, and that the continuous hot mill, used herein for purposes of illustration, is an example without limitation.

It should be understood, therefore, that many modifications and variations of the present invention may be made, which, though departing from the letter of this specification, will not be without the spirit and scope of the invention as are more particularly apprehended in and by the appended claim.

We claim:

The method of treating steel strip which includes: hot-reducing a strip to gauge to nish within the austenitic temperature range; quenching portions of the strip to a relatively low temperature hardening them to a hardness requiring tempering, while leaving other portions thereof at a temperature permitting hardening by quenching, and, then, immediately coiling said strip and causing tempering of the quenched sections and hardening of these hotter sections.

PATRICK H. HUME. WILLIAM F. MCGARRITY. EDWARD L. ROBINSON. 

